1. Field of the Invention
The present invention relates to a wiring structure including a wiring of a transparent conductive film, which is disposed, for example, on a display panel of a liquid crystal display device or the like.
2. Description of the Background Art
Recently, as a liquid crystal mode capable of achieving both characteristics of a wide viewing angle and high transmittance, adoption of an FFS (Fringe Field Switching) mode to a liquid crystal display device has been rapidly spread. An FFS mode TFT array substrate includes a structure in which two layers of transparent conductive films are overlapped with an insulating film therebetween, as shown, for example, in Japanese Patent No. 3826217. On the other hand, a transparent conductive film included in a general TN (Twisted Nematic) mode TFT array substrate is a single layer. Accordingly, the number of photoengraving steps required for manufacturing the FFS mode TFT array substrate is increased by at least one step compared to that of the general TN mode TFT array substrate.
As a measure against such increase in the number of steps, for example, Japanese Patent Application No. 2010-191410 discloses a technique of reducing the number of necessary photoengraving steps by devising the arrangement of a transparent conductive film pattern provided on the FFS mode TFT array substrate. The configuration of the FFS mode TFT array substrate disclosed in Japanese Patent Application No. 2010-191410 can be formed by using the same number of photoengraving steps as that in the manufacture of the general TN mode TFT array substrate.
As described above, the FFS mode TFT array substrate includes a structure in which the two layers of transparent conductive films overlap with the insulating film interposed therebetween. In this case, a balance between stress generated in the lower transparent conductive film and stress generated in the insulating film thereabove may not be kept, and this may cause a phenomenon called “film floating” or “film peeling” (hereinafter, collectively referred to as “film floating”) in which the insulating film of the upper layer is peeled at an end of the transparent conductive film or the like.
The frequency of occurrence of this film floating is associated with a pattern density of the transparent conductive film, and is increased on a region with a relatively sparse pattern density of the transparent conductive film, such as a frame region on the outside of a display region on the TFT array substrate, for example, an external connection terminal part, a wiring conversion part, or the like. Since a function of the insulating film as a protective film is lost on the portion where the film floating of the insulating film occurs, the film floating leads to reduction in corrosion resistance of an electrode, occurrence of dielectric breakdown, or the like, which causes reduction in a yield on the manufacture of the TFT array substrate and reduction in reliability of the TFT array substrate. Accordingly, in order to obtain a TFT array substrate having a high yield and high reliability, it is effective to take a measure against the film floating of the insulating film.
In order that the number of photoengraving steps of the FFS mode TFT array substrate may be made identical to that of the general TN mode TFT array substrate, Japanese Patent Application No. 2010-191410 employs a configuration in which a transparent conductive film is arranged on a metal film pattern serving as a source wiring without an insulating film interposed therebetween. In this configuration, since the area of the transparent conductive film pattern on the TFT array substrate is large, and increase in the frequency of occurrence of the film floating of the insulating film is concerned, the measure against the film floating becomes further important.
Moreover, in the TFT array substrate of Japanese Patent Application No. 2010-191410, since the metal film and the transparent conductive film are required to function as the electrically same electrode or wiring, the electrical connection between the metal film and the transparent conductive film is important. For example, since excellent electrical connection cannot be easily obtained between a transparent electrode such as ITO (Indium Tin Oxide) and Al, a problem arises when an ITO transparent conductive film is required to be disposed on a laminated film having an Al-based metal as an uppermost layer. The present inventor has confirmed that in a liquid crystal display device employing a TFT array substrate having a wiring structure in which ITO is arranged on Al, the resistance of the wiring conversion part that connects both wirings on different layers of the TFT array substrate increases, and a display defect such as a line defect occurs.
On the other hand, since an Al-based thin film has a low resistance value, the application of the Al-based thin film to a signal wiring of the TFT array substrate has become widespread with increase in resolution and increase in size of a screen of the liquid crystal display device. Therefore, in the development of the FFS mode TFT array substrate, it is important to improve the electrical connection between the transparent conductive film and the metal film, particularly, between the transparent conductive film and the laminated film having the Al-based thin film as the upper surface.